Heretofore, an image pickup element such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor to which a semiconductor microfabrication technology is applied has been widely adopted in a digital camera, a mobile phone and the like.
In such an image pickup element, in a photodiode provided within a pixel, light made incident to the photodiode from a subject is subjected to photoelectric conversion. A voltage signal corresponding to an amount of resulting electric charges is read out through an amplifying transistor and a vertical signal line, and is then subjected to AD (Analog to Digital) conversion by a comparator (for example, refer to PTL 1).